FIG. 6 is a block diagram illustrating a basic electrical structure of a dropper-type DC regulated power supply 1. The DC regulated power supply 1 is a three-terminal regulator with a single output, in which a power transistor q is interposed in series on a power line 1 that connects a power supply with a load (neither is shown). The base current of the power transistor q is controlled by a control IC ctl. In this way, an output voltage vo is adjusted to an output voltage level that has been set beforehand according to the load requirement, thereby generating a stable DC voltage.
In the control IC ctl, a differential amplifier a compares a reference voltage vref from a reference voltage source 2 with a feedback voltage vadj, the reference voltage source 2 being provided either internally or externally to the control IC ctl (internally in the example shown in FIG. 6), and the feedback voltage vadj being produced by dividing the output voltage vo through resistors r1, r2. By thus comparing the two voltages, the differential amplifier a controls the base current of the power transistor q via a control transistor tr, so as to match the two voltages. On the output side of the power transistor q is provided an output capacitor (not shown) for smoothing the output.
The DC regulated power supply 1 with the described structure generally includes a protection circuit, such as an overcurrent protection circuit b and an overheat protection circuit 3, by which the base current of the power transistor q is reduced when an overcurrent flows or when there is a temperature increase in the series regulator, so as to protect the DC regulated power supply 1.
However, the DC regulated power supply 1 has a drawback in that the operation of the protection circuit influences other output systems when the output of more than one system is packed and sealed in a single package. This is due to the fact that the DC regulated power supply 1 is a single-output regulator, whereby the circuits of each functional block are designed for single output.
FIG. 7 is a block diagram showing an electrical structure of a DC regulated power supply 11 with outputs of multiple systems (two systems in FIG. 7). The DC regulated power supply 11 is realized by connecting a plurality of DC regulated power supplies 1 of the single-output structure in parallel, using a common input. In FIG. 7, the same reference numerals are used for the constituting elements that were already described with reference to FIG. 6, and like elements are indicated by adding numbers.
In the DC regulated power supply 11 having the foregoing structure, upon detecting an overcurrent state, overcurrent protection circuits b1, b2 bypass the base currents that are respectively supplied from differential amplifiers a1, a2, so as to control transistors tr1, tr2. This reduces the base currents of the corresponding power transistors q1, q2, thereby carrying out the protection operation.
Similarly, when an overheated state is detected in the overheat protection circuit 3, the base currents supplied to the control transistors tr1, tr2 are bypassed to reduce the base currents of the power transistors q1, q2, thereby carrying out the protection operation.
Thus, in the DC regulated power supply 11, reducing the base currents by the overheat protection circuit 3 in an overheated state effects the overheat protection operation only in one of the systems, owning to differences in circuit constant or load condition, i.e., differences in threshold of the transistors due to different values of the base currents, which shifts the temperature (timing) of reducing the base currents from each other.
Here, fluctuations of the driving current causes the driving current of the other system to fluctuate, causing the output voltage to fluctuate. Particularly, when the output voltage increases and exceeds an acceptable voltage range of the load, the load may be damaged.
The overheat protection circuits of the DC regulated power supply 11 operate when the temperature exceeds an overheat protection temperature. Below the overheat protection temperature, the overheat protection circuits do not operate and the DC regulated power supply 11 carries out normal regulation operation.
As described above, the overheat protection circuits of the DC regulated power supply 11 operate at different timings. Accordingly, when the overheat protection circuits operate in response to generated heat of the regulator and when the overheat protection operation becomes effective only in one of the systems under a maintained device temperature in the vicinity of the overheat protection temperature, the output of the protected system may place the other system under the influence of driving current fluctuations for an extended period of time.
Such a phenomenon is especially prominent when one of the systems has an excessively small output current and the overheat protection circuit operates in response to generated heat caused by the output current of the other system. FIG. 8 describes how this gives rise to malfunctions, with reference to respective waveforms of chip temperature, output voltage, output current, and input current.
When the output current of one system is excessively small, the overheat protection circuit of the other system that has generated heat operates and reduces only the current of the output that generated heat. Here, when the output current becomes temporarily dull, the input current also becomes dull if the capacity of the power supply is not sufficient. The output with excessively small output current becomes more susceptible to the influence of the dull input current as the current becomes smaller. This may result in regulation control failure or regulator malfunction. In the worst case, the output voltage vo increases above the pre-set voltage to cause damage to the load devices. Thus, the multi-output regulator needs to be designed such that the operation of one system does not affect the operations of the other systems.
This is particularly true in one-chip configuration. Depending on the layout of a semiconductor circuit block, operations of the control IC or protection circuit of one system may cause potential fluctuations due to heat transfer or current flow even when the other systems are not operating. This becomes more likely as the circuit distance within the chip becomes shorter. Further, even when the circuit design is the same, a common circuit component may cause differences in circuit constant due to a distance therefrom or due to asymmetricity of the circuit. It is therefore required that the circuits be designed to minimize the influence of one system over another.